Fiber optic circuitry is increasingly being used in electronics systems where circuit density is ever-increasing and is difficult to provide with known electrically wired circuitry. An optical fiber circuit is formed by a plurality of optical fibers carried by a dielectric, and the ends of the fibers are interconnected to various forms of connectors or other optical transmission devices. A fiber optic circuit may range from a simple cable which includes a plurality of optical fibers surrounded by an outer cladding or tubular dielectric to a more sophisticated optical backplane or flat fiber optic circuit formed by a plurality of optical fibers mounted on a substrate in a given pattern or circuit geometry.
One type of optical fiber circuit is produced in a ribbonized configuration wherein a row of optical fibers are disposed in a side-by-side parallel array and coated with a matrix to hold the fibers in the ribbonized configuration. In the United States, a twelve-fiber ribbon or an eight-fiber ribbon have become common. In other foreign countries, the standard may range from as a low as four to as high as twenty-four fibers per ribbon. Multi-fiber ribbons and connectors have a wide range of applications in fiber optic communication systems. For instance, optical splitters, optical switches, routers, combiners and other systems have input fiber optic ribbons and output fiber optic ribbons.
With various applications such as those described above, the individual optical fibers of input fiber optic ribbons and output fiber optic ribbons are cross-connected or reorganized whereby the individual optical fibers of a single input ribbon may be separated and reorganized into multiple or different output ribbons. The individual optical fibers are cross-connected or reorganized in what has been called a “mixing zone” between the input and output ribbons.
Optical backplanes are fabricated in a variety of manners, ranging from laying the optical fibers on a substrate by hand to routing the optical fibers in a given pattern or circuit geometry onto the substrate by mechanized apparatus. The individual optical fibers are cross-connected or reorganized on the substrate between input and output ribbons projecting from input and output ends or edges of the substrate. Therefore, the above-mentioned “mixing zone” is provided by the substrate, itself.
When cross-connecting optical fibers on substrates of optical backplanes, problems often are encountered because of the space limitations of a given application. In other words, the size of the substrate may be limited, but it may be necessary in a given specification to have more individual optical fibers in the input and output ribbons than the limited sized substrate can handle. This is particularly true when fabricating the optical backplane by mechanized apparatus. For example, a routing apparatus with a routing head may require 4-5 mm both in front of and behind a first laid optical fiber for laying a second fiber over the top of the first fiber. This requires an amount of space or “real estate” on the substrate. Obviously, if a substrate has a limited size, only a limited number of individual optical fibers can be cross-connected or reorganized on the substrate, and this limited number of individual fibers may be insufficient to fabricate input and output ribbons to meet a particular specification. Consequently, it may be necessary to fabricate a layered backplane wherein one or more substrates (with their limited number of routed fibers) are stacked on top of another substrate (with its limited number of routed fibers), whereby the fibers of the stacked substrates are combined to form the specified input and output ribbons.
Heretofore, multi-layered backplanes or circuits have been fabricated by placing a bottom layer adhesive coated substrate on a base sheet of adhesive coated paper-like material on top of a flat table or other platform. Individual optical fibers are placed on the bottom layer substrate and base sheet, with the fibers projecting beyond edges of the substrate to form ribbon tails. A conformal coating is applied to the bottom layer substrate and fibers, and the coating is cured. A second or top layer substrate is placed on top of the bottom layer, and individual optical fibers again are placed on the top layer substrate with end portions of the fibers extending outwardly onto the base sheet to form ribbon tails. A conformal coating is applied to the top layer substrate and all of the ribbon tails, and the coating is cured. The layered substrate and ribbon tails then are peeled off of the base sheet, and the base sheet is discarded. Such methods or processes can only be made by hand and, typically, one operator follows the entire process from start to finish for consistency reasons. A double-layered backplane may take as long as a full 8-hour day to complete. In addition, hand routing or laying of the fibers is difficult for maintaining straight lines and uniform ribbon tails with the tiny individual optical fibers.
The present invention is directed to solving these various problems in a method of manufacturing a multi-layer backplane or optic circuit which is particularly applicable for mechanized fabrication and involves the use of a simple ribbonizing apparatus.